This invention generally relates to self-excitation of a switched reluctance generator during a short-circuit or load bus fault without the need for a battery or auxiliary DC power source.
Switched reluctance electric generators are often used in aerospace applications, where they typically provide electrical power for the vehicle in which they reside. In such aerospace applications, a generator must be able to provide current during short-circuit and fault conditions on a positive load bus.
A load fault can occur when the maximum load current for a particular generator system is exceeded. A common type of load fault is a short circuit, which is an accidental low-resistance connection between two nodes of a circuit that are meant to be at different voltages. A load fault would typically happen not in the generator itself but somewhere in the load that it is powering. If a fault occurs, the output voltage of the generator drops to zero. During such a load fault, current from the generator is still needed to operate protective devices such as circuit-breakers and fuses.
Switched reluctance electric generators also require excitation energy to operate. This excitation energy must come from a DC power source. Since a switched reluctance generator itself is a source of DC power, the generator typically relies on its own DC output as a source of excitation energy. This is known as self-excitation, because the device is producing its own excitation energy.
In the event of a short-circuit or load fault, the generator still requires excitation energy to provide current for protective devices, however because of the short-circuit or fault condition, the generator can no longer use its DC output as a source of this energy. In such situations, a switched reluctance generator needs an alternate source of excitation energy to keep operating. Without the necessary excitation energy to sustain the generator in the event of a fault, the entire system would simply discharge and stop producing power. However, if a source of excitation energy is present during a fault, the generator can continue producing current and can return to its normal voltage and resume normal operation.
A conventional solution to this need for excitation energy involves a capacitor bank connected to a positive and negative bus of the generator. In this setup, the generator takes a small amount of energy from the capacitor bank for excitation during each electrical cycle. In the event of a fault on a load bus, this capacitor bank will completely discharge. A drawback of this conventional setup is that once the capacitor bank discharges, it can no longer excite the switched reluctance generator to operate, because the capacitor bank has been discharged to zero volts.
Another solution to this problem involves maintaining a separate excitation bus connected to an external power source, such as a battery or other auxiliary generator in the electrical system. Although this overcomes the earlier problem, it introduces additional problems. One drawback to this solution is overall diminished reliability due to the increased complexity and the need for additional hardware. Another drawback is an increase in the overall weight of the generator, which is undesirable in aerospace applications, due to the additional power source.
It is therefore the objective of this invention to provide a novel scheme for self-excitation of a switched reluctance generator in the event of a load bus fault, which does not require a battery or auxiliary generator in the electrical system.